The present invention relates to a method and circuit for detecting a light load mode of a switching power supply system, and more particularly to the method and circuit for detecting a light load mode of a switching power supply system by using an assistant winding.
Switching power supply systems are generally used in electrical apparatuses, such as the computer systems or the measuring systems, for providing a stable direct current (DC). Therefore, a switching power supply system having a light load and power saving mode has been developed because of the severe standard of environment protection gradually, such as FCC and other European standard. Consequently, the power supply system capable of selectively entering the light load and saving power mode would be more important to fabricate the electrical apparatuses.
Nowadays, a power saving mode is provided in many electrical apparatuses for avoiding the unnecessary power consumption from the power supply system and increasing the life-time of the power supply system when the system load is excessively light. Furthermore, the method for switching the system to a light load and power saving mode is performed by an application specific integrated circuit (ASIC) to switch the system operation mode such as switching to the related power saving circuit or reducing the switching frequency according to the conventional switching power supply system.
In the above-mentioned description, the conventional method for switching the light load and power saving mode is implemented by detecting a feedback voltage signal change of the control circuit via the application specific integrated circuit (ASIC). However, the method has the following drawbacks:
1. The specific IC material such as ASIC could be easily deficient to increase the cost thereof; and
2. The feedback voltage signal change is too small to be effective, and a vibration and a noise may be generated when switching is under the high load.
It is therefore tried by the applicant to deal with the above situation encountered by the prior art.
It is therefore an object of the present invention to provide a method and a circuit for detecting a light load mode of a switching power supply system by employing an assistant winding to determine whether the switching power supply system is under a light load mode to activate a power saving circuit.
According to an aspect of the present invention, there is provided a method for detecting a light load mode of a switching power supply system, wherein the switching power supply system has a first winding, a second winding, a third winding having a voltage being in proportion to an output voltage of the second winding, and a signal converting device electrically connected to the third winding. The method includes steps of measuring a voltage of the signal converting device and determining the switching power supply system being under the light load mode when the voltage of the signal converting device is smaller than a predetermined threshold voltage.
Preferably, the third winding is an assistant winding, and an outputted signal from the assistant winding is processed via the signal converting device and provided for determining whether the switching power supply system is under said light load mode.
Preferably, the first winding and the second winding are respectively a primary winding and a secondary winding of a transformer in the switching power supply system.
Preferably, the switching power supply system further includes a detector electrically connected to the signal converting device for determining whether the switching power supply system is under the light load mode.
Preferably, the method further includes a step of activating a power saving circuit when the switching power supply system is under the light load mode.
According to another aspect of the present invention, there is provided a method for activating a power saving circuit of a switching power supply system in response to the magnitude of a load, wherein the switching power supply system has a first winding, a second winding connected to the load, a third winding having a voltage being in proportion to an output voltage of the second winding, and a signal converting device electrically connected to the third winding. The method includes steps of measuring a voltage of the signal converting device and determining whether the switching power supply system is under a light load mode, outputting a power saving signal representative of the light load mode when the output voltage of the signal converting device is smaller than a predetermined threshold voltage, and activating the power saving circuit in response to the power saving signal.
According to another aspect of the present invention, there is provided the circuit for detecting a light load mode of a switching power supply system, wherein the switching power supply system has a first winding, a second winding, and a third winding having a voltage being in proportion to an output voltage of the second winding. The circuit includes a signal converting device electrically connected to the third winding for generating a voltage in response to the magnitude of a load which is connected to the second winding and a detector electrically connected to the signal converting device for outputting a signal representative of the light load mode when the voltage of the signal converting device is smaller than a predetermined threshold voltage. According to another aspect of the present invention, the switching power supply system includes a first winding, a second winding, a third winding having a voltage being in proportion to an output voltage of the second winding, a signal converting device electrically connected to the third winding for generating a voltage variation in response to the magnitude of a load which is connected to the second winding, a detector electrically connected to the signal converting device for outputting a power saving signal when the voltage of the signal converting device is smaller than a predetermined threshold voltage, and a power saving circuit for receiving the power saving signal for performing a power saving mode
The present invention may best be understood through the following descriptions with reference to the accompanying drawings, in which: